Access to the underlying UIWindow & NSWindow in swiftUI gives access to window methods and attributes.

ExampleWindowReaderApp.swift

//
//  ExampleWindowReaderApp.swift
//  Shared
//
//  Created by Matthaus Woolard on 21/12/2020.
//

import SwiftUI

@main
struct ExampleWindowReaderApp: App {
    var body: some Scene {
        WindowGroup {
            ContentView().modifier(WindowObservationModifier())
        }
    }
}

#if canImport(UIKit)
    typealias Window = UIWindow
#elseif canImport(AppKit)
    typealias Window = NSWindow
#else
    #error("Unsupported platform")
#endif


class WindowObserver: ObservableObject {
    
    @Published
    public private(set) var isKeyWindow: Bool = false
    
    private var becomeKeyobserver: NSObjectProtocol?
    private var resignKeyobserver: NSObjectProtocol?

    weak var window: Window? {
        didSet {
            self.isKeyWindow = window?.isKeyWindow ?? false
            guard let window = window else {
                self.becomeKeyobserver = nil
                self.resignKeyobserver = nil
                return
            }
            
            self.becomeKeyobserver = NotificationCenter.default.addObserver(
                forName: Window.didBecomeKeyNotification,
                object: window,
                queue: .main
            ) { (n) in
                self.isKeyWindow = true
            }
            
            self.resignKeyobserver = NotificationCenter.default.addObserver(
                forName: Window.didResignKeyNotification,
                object: window,
                queue: .main
            ) { (n) in
                self.isKeyWindow = false
            }
        }
    }
}

extension EnvironmentValues {
    struct IsKeyWindowKey: EnvironmentKey {
        static var defaultValue: Bool = false
        typealias Value = Bool
    }
    
    fileprivate(set) var isKeyWindow: Bool {
        get {
            self[IsKeyWindowKey.self]
        }
        set {
            self[IsKeyWindowKey.self] = newValue
        }
    }
}

struct WindowObservationModifier: ViewModifier {
    @StateObject
    var windowObserver: WindowObserver = WindowObserver()
    
    func body(content: Content) -> some View {
        content.background(
            HostingWindowFinder { [weak windowObserver] window in
                windowObserver?.window = window
            }
        ).environment(\.isKeyWindow, windowObserver.isKeyWindow)
    }
}

#if canImport(UIKit)
struct HostingWindowFinder: UIViewRepresentable {
    var callback: (Window?) -> ()

    func makeUIView(context: Context) -> UIView {
        let view = UIView()
        view.translatesAutoresizingMaskIntoConstraints = false
        DispatchQueue.main.async { [weak view] in
            self.callback(view?.window)
        }
        return view
    }

    func updateUIView(_ uiView: UIView, context: Context) {
    }
}
#elseif canImport(AppKit)
struct HostingWindowFinder: NSViewRepresentable {
    var callback: (Window?) -> ()

    func makeNSView(context: Self.Context) -> NSView {
        let view = NSView()
        view.translatesAutoresizingMaskIntoConstraints = false
        DispatchQueue.main.async { [weak view] in
            self.callback(view?.window)
        }
        return view
    }
    func updateNSView(_ nsView: NSView, context: Context) {}
}
#else
    #error("Unsupported platform")
#endif

I add this code to my project and has a error? any idea why? The error below.
Undefined symbols for architecture x86_64:
"(extension in Poke):SwiftUI.EnvironmentValues.isKeyWindow.setter : Swift.Bool", referenced from:
key path setter for (extension in Poke):SwiftUI.EnvironmentValues.isKeyWindow : Swift.Bool : SwiftUI.EnvironmentValues in PokeApp.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

@xiaoxidong That is a strange error to get. What version of Xcode are you running? And have you cleaned your build folder?

Same to me on Xcode 12.4

@xwischi does this package build and work? https://github.com/LostMoa/KeyWindow

It's probably the fileprivate(set) var isKeyWindow: Bool ... part